Title: Chronotherapy as a Strategy to Attenuate Toxicity Associated with Cisplatin and Radiation Treatment for Triple-Negative Breast Cancer PROJECT SUMMARY/ABSTRACT Triple-negative breast cancers (TNBCs) constitute 10-20% of all breast cancers, more frequently inflict young patients, and are more prevalent in African American women. According to ClinicalTrials.gov, there are currently 8 active clinical trials underway for treatment of TNBC patients with cisplatin followed by radiation therapy. Although cisplatin is an effective chemotherapeutic agent for a wide variety of tumors, the dose-limiting toxicity, specifically renal failure, precludes optimal therapeutic outcomes. Additionally, radiation therapy/ionizing radiation is a common therapeutic modality for treating TNBCs, particularly in combination with cisplatin. Despite technological advances, major adverse effects of radiation therapy include inflammation, radio-resistance, and tumor relapse. Minimizing toxicity is critical to improving the effectiveness of cisplatin and radiation therapies against TNBCs. Chronotherapy ? the administration of treatment at specific times of the day to maximize efficacy or minimize toxicity ? was introduced over 30 years ago, but its impact on clinical practice has been limited due to a lack of understanding of the underlying molecular mechanisms. The long-term goal of this work is to better understand how the circadian clock plays a role in increasing the efficacy of cisplatin and/or radiation treatments in cancer patients. The objective of this proposal is to determine the mechanistic basis of chronotherapy to minimize toxicities and improve cisplatin and radiation treatment efficacy in TNBC patients. Previously published data from our group and the preliminary data within this proposal show that DNA damage response (DDR) signaling events are controlled by the circadian clock. We hypothesize that cisplatin and/or radiation treatment- mediated toxicity and tumor shrinkage are regulated by the circadian clock. This may result in tumor cells that are more vulnerable to these genotoxic stress agents at specific times of the day, when healthy tissues are resistant to these agents. The following specific aims will be pursued: 1) Determine how the circadian rhythm impacts platinum drug-induced toxicity, pharmacodynamics, and pharmacokinetics in healthy tissues and TNBC tumors of genetic mouse models. 2) Identify the role of the circadian clock in ionizing radiation-induced DDR signaling pathways and toxicity in healthy breast and TNBC tumors of genetic mouse models. 3) Characterize the impact of circadian time on cisplatin and ionizing radiation treatment efficacy against a TNBC-prone genetic mouse model and patient-derived xenograft (PDX) mouse model using primary tumor biopsies from TNBC patients. This approach is innovative because it combines recently developed mouse models and PDX tumor models to advance the mechanistic understanding of how the circadian rhythm can be harnessed to improve efficacy and minimize toxicity of cisplatin and radiation treatment separately or in combination for the treatment of TNBCs. Completion of these aims will significantly contribute to understanding circadian time-moderated therapies on healthy tissue toxicity, as well as tumor shrinkage, at the mechanistic level and determine whether time of day has a protective role against cisplatin and radiation treatments alone or in combination. We also seek to test these chronotherapies on both healthy and circadian-disrupted animals to determine necessary treatment differences for individuals with abnormal sleep-wake cycles (e.g., shift workers).